A light-emitting element having a layer containing a luminescent substance between a pair of electrodes, which is used as a pixel, a light source, or the like, is provided for a light-emitting device such as a display device or a lighting system. When current flows between the pair of electrodes in the light-emitting element, fluorescence or phosphorescence is emitted from an excited luminescent substance.
In comparison with fluorescence, theoretically, internal quantum efficiency of phosphorescence is 3 times as much as that of fluorescence in the case of current excitation. Therefore, it is considered that higher luminous efficiency is obtained by using a luminescent substance emitting phosphorescence than using a luminescent substance emitting fluorescence; thus, a substance emitting phosphorescence has been developed.
For example, a metal complex where central metal is iridium is mentioned in Reference 1: Japanese Patent Application Laid-Open No. 2001-247859. According to the reference, this metal complex can be used as a material for a light-emitting element.